The term "foam expansion" is well understood in the art. A low-expansion type foam is one which has a liquid to foam ratio of less than 1:20 (&lt;1:20); a medium-expansion type foam has a ratio of from 1:20 to 1:250 and a high-expansion type foam has a ratio greater than 1:250(&gt;1:250), ideally not exceeding 1:1000.
Modern safety standards in the industrial environment and the increasing and ever more complex fire risks have necessitated the installation of fire fighting equipment. This would typically consist of fire blankets and fire extinguishers, of for example, water, foam, carbon dioxide, halon and powder types.
Portable fire extinguishers are required in all sectors of industry but are particularly vital in high risk areas such as chemical installation, laboratories, petrol stations, power stations, kitchens, oil rigs etc. The usefulness of the present invention can be seen by considering deep fat fires in kitchens, especially of fast food outlets where the easy and safe extinguishing of those fires is rather difficult to achieve using conventional extinguishers.
Carbon dioxide can be effective on many fires, but for use on burning fat or cooking oil it has the disadvantage that although it extinguishes the flames, it does not cool the fat or oil sufficiently to bring it below its auto-ignition point. Hence, when the carbon dioxide discharge is stopped, re-ignition of the fat or oil is inevitable.
Dry powders are also effective on many fires, including fats and oils, but it has the disadvantage that it is very messy, does not cool sufficiently and evolves acrid fumes in many cases. The discharges from powder extinguishers are also very powerful and can splash the hot fat or oil onto the surroundings.
Halon 1211 (Bromochlorodifluoromethane--BCF) extinguishers are not effective on deep fat fires and their use on such fires tends to produce a cocktail of toxic gases. Also, Halons pose a serious threat to the ozone layer and their use is diminishing.
Water in any form from an extinguisher is not suitable for use on deep fat fires.
Previously known portable foam extinguishers have used only low-expansion foams or have been of the non-aspirated spray type. Although both forms of foam extinguishers are well established and are effective when used by skilled operators, there are several problems associated with their use.
Low-expansion foams used in presently available types of fire extinguishers are actually relatively "wet" as the ratio of air to water is not particularly high. When used in fighting fires these foams can cause significant water damage. Another consequence of the water content of the low-expansion foam is the risk, for example, in fighting a deep fat fire in a kitchen, of causing the fat to spit or erupt and boil over violently before the fire is extinguished. This is a significant hazard to the operator of the fire extinguisher and can also cause the fire to spread.
Potential operators of low-expansion foam fire extinguishers require special training and practice to ensure safe and effective use of the apparatus. Even skilled operators may cause damage to fragile equipment due to the high velocity of the discharge from the extinguishers. If the low-expansion foam is applied incorrectly to a flammable liquid fire, the foam may be driven under the surface of the burning liquid and rendered ineffective.
Foam is perhaps the best agent currently available for deep-fat fires in kitchens. However, it is seldom selected due to the disadvantages described above. In particular, the fact that incorrect application can cause boil-over and fire-ball effects leading to the fire spreading and to injuring the operator, has mitigated against the wider use of foam fire extinguishers in industrial cooking areas.
Medium expansion type foams are well known in the art for extinguishing fires involving flammable liquids and are of particular use in fighting fires in confined areas such as cellars and engine rooms on ships etc. They can also be used to secure spills of flammable liquids and to suppress fume release from toxic spills etc. Although they are often used on a large scale by fire brigades, ship fire crews, petrochemical plants etc, it has not previously been possible to use these foams on smaller fires due to the bulky nature of the foam generating equipment and their high delivery rates. The most common medium expansion foam generator in current use, often referred to as a "foam branch pipe", requires a foam compound solution supply rate of 450 liters/min at 4 bar (4.times.10.sup.5 Pa) pressure. This device will deliver on average 7,000 gallons of foam per minute (26.5m.sup.3 /min).